It is composed
of rocks, unlike any which I have met with, and which I cannot characterise
by any name, and must therefore describe.

The simplest, and one of the most abundant kinds, is a very compact, heavy,
greenish-black rock, having an angular, irregular fracture, with some
points just hard enough to scratch glass, and infusible. This variety
passes into others of paler green tints, less hard, but with a more
crystalline fracture, and translucent on their edges; and these are fusible
into a green enamel. Several other varieties are chiefly characterised by
containing innumerable threads of dark-green serpentine, and by having
calcareous matter in their interstices. These rocks have an obscure,
concretionary structure, and are full of variously coloured angular pseudo
fragments. These angular pseudo fragments consist of the first-described
dark green rock, of a brown softer kind, of serpentine, and of a yellowish
harsh stone, which, perhaps, is related to serpentine rock. There are other
vesicular, calcareo-ferruginous, soft stones. There is no distinct
stratification, but parts are imperfectly laminated; and the whole abounds
with innumerable veins, and vein-like masses, both small and large. Of
these vein-like masses, some calcareous ones, which contain minute
fragments of shells, are clearly of subsequent origin to the others.

A GLOSSY INCRUSTATION.

Extensive portions of these rocks are coated by a layer of a glossy
polished substance, with a pearly lustre and of a greyish white colour; it
follows all the inequalities of the surface, to which it is firmly
attached. When examined with a lens, it is found to consist of numerous
exceedingly thin layers, their aggregate thickness being about the tenth of
an inch. It is considerably harder than calcareous spar, but can be
scratched with a knife; under the blowpipe it scales off, decrepitates,
slightly blackens, emits a fetid odour, and becomes strongly alkaline: it
does not effervesce in acids. (In my "Journal" I have described this
substance; I then believed that it was an impure phosphate of lime.) I
presume this substance has been deposited by water draining from the birds'
dung, with which the rocks are covered. At Ascension, near a cavity in the
rocks which was filled with a laminated mass of infiltrated birds' dung, I
found some irregularly formed, stalactitical masses of apparently the same
nature. These masses, when broken, had an earthy texture; but on their
outsides, and especially at their extremities, they were formed of a pearly
substance, generally in little globules, like the enamel of teeth, but more
translucent, and so hard as just to scratch plate-glass. This substance
slightly blackens under the blowpipe, emits a bad smell, then becomes quite
white, swelling a little, and fuses into a dull white enamel; it does not
become alkaline; nor does it effervesce in acids. The whole mass had a
collapsed appearance, as if in the formation of the hard glossy crust the
whole had shrunk much. At the Abrolhos Islands on the coast of Brazil,
where also there is much birds' dung, I found a great quantity of a brown,
arborescent substance adhering to some trap-rock. In its arborescent form,
this substance singularly resembles some of the branched species of
Nullipora. Under the blowpipe, it behaves like the specimens from
Ascension; but it is less hard and glossy, and the surface has not the
shrunk appearance.